In heat engine applications, a large amount of low temperature heat is produced and has to be dissipated into the environment, typically through wet-cooling or dry-cooling methods. Since heat engines typically use high temperature high pressure vapor (e.g., water vapor/steam) expansion to generate power, this waste heat is typically provided as a low temperature vapor with latent heat. In wet-cooling processes, fresh cooling water (e.g., from a water tower) is utilized in a condenser to remove the low temperature latent heat by dissipating large amounts of latent heat in the cooling water thereby increasing the cooling water temperature, and eventually dissipating the heat into the environmental air. In dry-cooling processes, large wind blowers are typically used with much larger heat exchanger(s) because air does not as efficiently dissipate the latent heat as water. This latent heat that is dissipated often accounts for more than 50% of the total energy consumed in thermal power generation stations worldwide, including fossil fuel based fire power plants, concentration solar power plants (CSPs), and nuclear power plants.
Some important direct thermal energy applications, such as absorption heat pumps and low temperature multi-effect evaporation desalination processes, utilize mid and low temperature water vapor. Normally a large amount of thermal energy has to be consumed to obtain the mid and low temperature vapor because vaporization processes require large amounts of latent heat during the phase change process. If absorption heat pumps and absorption heat transformers can be made so that mid to low temperature residue latent heat energy can be further utilized instead of condensed into water directly via wet-cooling or dry-cooling methods, a significant amount of thermal energy can be saved and a great amount of carbon dioxide emissions can be reduced.
However, in typical conventional absorption heat pumps (so called first type heat pumps), a high temperature heat source is needed to generate high enough temperature vapor to convert the mid to low temperature latent heat from the mid to low temperature vapor into higher temperature water for space heating (or other) purposes. Although these systems can utilize the mid to low temperature latent heat of the vapor, the sensible heat produced in the higher temperature water is less valuable as compared to a similar temperature vapor. This is because vapor forms of thermal energy are readily used in low temperature multi-effect evaporation processes, e.g., for water purification and/or desalination processes. Therefore, a method of producing higher temperature thermal energy in vapor form with type one heat pumps is of interest.
On the other hand, mechanical vapor recompression (MVR) is an effective method that utilizes a mechanical compressor to compress and heat “waste” vapor to a higher temperature for reuse. For example, MVR is used in drying processes where concentrated inorganic solutions from, e.g., the end stage of sewage processing are dried to powder so that no pollutants are released into the environment. The conventional drying process consumes a great amount of thermal energy because the water has to be heated to boiling point to be vaporized and this phase change process requires a lot of latent heat. Using MVR vapor from the drying process can be recompressed to a higher temperature so that this higher temperature vapor can be reused to vaporize water from the solution with a heat exchanger. In this process, a certain amount of electricity is consumed to drive the mechanical compressor. In other words, electrical energy is converted into sensible heat of the vapor and thereby increases the vapor temperature. If a vapor driven absorption heat pump and absorption heat transformer (also called second type heat pump) could be used adequately, it would be possible to consume the thermal energy to recompress the used vapor. This approach would save high quality energy, such as electricity, and provide an economic benefit because electrical energy is more expensive than the thermal energy.
Furthermore, in a conventional generator for a LiBr solution based heat pump, the heat exchanger utilizes “pool boiling” to generate vapor from a diluted LiBr solution in order to concentrate the LiBr solution for absorption usage. This is because in most cases, the heat exchanger in the generator has a regular tube/shell configuration with the tubes that connects with the heat source immersed into the LiBr solution. The temperature difference between the heat source and the diluted LiBr solution should be more than 20 degrees in order to sufficiently generate water vapor. Using a conventional thermal energy source, such as a nature gas fire boiler, this temperature and pressure requirements pose no constrains for the heat source what so ever. This is because for a fossil fuel heat source, a mere 20 to 30-degree temperature difference in vapor generation process will not consume much more fossil fuel, and the boiler works at much higher temperature anyway.
However, if the heat comes from different sources, such as solar thermal collectors or waste heat from other heat engines or appliances, this 20 degree or higher temperature difference is hard to obtain. If lower temperature steam vapor can be utilized, the requirement for the heat source temperature can be lowered to drive the heat pump or absorption chiller.